In order to realize the primary goal, schedulers of wireless communication systems calculate allocation indexes corresponding to the channel quality, and allocate wireless resources according to those allocation indexes. As a technique relating to schedulers for the purpose of reducing power consumption, i.e. power saving (to be referred to as power saving schedulers hereinafter), there are CSDP (Channel State Dependent Packet) schedulers (see Nonpatent Documents 1 and 2).
In Nonpatent Documents 1 and 2, the receiver's received signal level Pr is utilized as the allocation index. The scheduler compares the receiver's received signal level Pr with a preset threshold value Pth. The scheduler determines the receivers with a received signal level Pr equal to or higher than the threshold value Pth to be in a ‘good state’. Further, the scheduler determines the receivers with a received signal level Pr lower than the threshold value Pth to be in a ‘bad state’ (see FIG. 20). The scheduler restrains wireless resources from allocation to the receivers according to the determination results. In Nonpatent Document 1, wireless resources are allocated preferentially to the receivers determined to be in the good state. In Nonpatent Document 2, wireless resources are allocated only to the receivers determined to be in the good state.
On the other hand, Nonpatent Document 3 has proposed a cross-layer scheduler taking congestion control into consideration. First, the cross-layer scheduler calculates the allocation index from the channel quality measured at the PHY layer (Physical layer), and the time margin up to allowable delay measured at the MAC layer (Media Access Control layer). Next, the cross-layer scheduler allocates wireless resources according to the allocation index calculated above. By virtue of this, the cross-layer scheduler realizes congestion control and throughput maximization simultaneously.
Further, Patent Document 1 discloses a method for scheduling wireless communication systems sending data from devices in wireless base stations by calculating indexes for selecting mobile stations based on the line quality between a plurality of mobile stations and the devices in the wireless base stations, and selecting mobile stations based on those indexes. Further, Patent Document 2 discloses a technique for allocating resources of communication systems by utilizing a plurality of allocation procedures including a low-speed allocation procedure and a high-speed procedure.
Patent Document 1: JP 2008-187449 A
Patent Document 2: Japanese Translation of PCT 2006-513632 A
Nonpatent Document 1: P. Bhaqwat, P. Bhattacharya, A. Krishna, and S. K. Tripathi, “Enhancing throughput over wireless LANs using channel state dependent packet scheduling,” IEEE Proc. INFOCOM'96, Vol. 3, pp. 1133-11 40, San Francisco, Calif., USA, March 1996.
Nonpatent Document 2: B. R. Badrinath and P. Sudame, “To send or not to send: implementing deferred transmissions in a mobile host,” Distributed Computing Systems, 1996., Proceedings of the 16th International Conference on, pp. 327-333, Hong Kong, May 1996.
Nonpatent Document 3: Y. J. Zhang and S. C. Liew, “Link-adaptive largest-weighted-throughput packet scheduling for real-time traffics in wireless OFDM networks,” in Proc. IEEE Global Telecommunications Conf., vol. 5, pp. 2490-2494, St. Louis, Mo., 2005.
According to the techniques described in Nonpatent Documents 1 and 2, because the transmitter has raised the probability of transmission to the receivers with a received signal level Pr equal to or higher than the threshold value Pth, it is possible to reduce the transmission power per transmission rate, thereby realizing power saving. However, because of the insufficient opportunity of allocating wireless recourses to the receivers determined to be in the bad state, in the case of a high degree of congestion, there is a problem that congestion will occur.
On the other hand, according to the technique described in Nonpatent Document 3, because of calculating the allocation index from the channel quality and the time margin up to allowable delay, it is possible to avoid congestion. However, with the technique described in Nonpatent Document 3, in the case of a low degree of congestion, because the allocation frequency is increased to the receivers with bad channel quality, the effect of power saving is small.
That is, as for Nonpatent Documents 1, 2 and 3, it is difficult to achieve both objectives of restraining congestion and saving power. On the other hand, in Patent Documents 1 and 2, there are no descriptions about any technique for achieving both objectives of restraining congestion and saving power.
Further, it is conceivable to formulate a method of utilizing the technique described in Nonpatent Document 3 to calculate the allocation index, comparing this calculated allocation index with the threshold value preset as described in Nonpatent Document 1 or 2, determining the receivers to be in either of the good state or the bad state, and restraining wireless resources from allocation to receivers according to the determination results. However, in such a method, because a fixed value is adopted for the threshold value compared with the allocation index, it is difficult to achieve both objectives of restraining congestion and saving power. The reason will be explained hereinbelow utilizing FIGS. 21A and 21B.
FIGS. 21A and 21B show an example of the relationship between the degree of congestion and the threshold value of channel quality allowing for wireless resource allocation. In FIGS. 21A and 21B, the number of packets waiting for transmission is denoted by the number of darkly painted squares. For example, the number of packets waiting for transmission to the receiver 12 is two. Receivers are aligned in the order of channel quality. The scheduler classifies the receivers with an allocation index equal to or higher than the threshold value into the good state based on the channel quality, and allocates wireless resources only to receivers in the good state.
In FIG. 21A, the threshold value is set to be low (the threshold value 1). When a low threshold value is set, wireless resources are allocated also to the receivers with unfavorable channel quality. Therefore, the effect of preventing the occurrence of congestion is increased. In the case of a large number of receivers waiting for transmission and a large number of packets each waiting for transmission, a low threshold value leads to a good result. However, as shown in FIG. 21A, even in the case of a small number of receivers waiting for transmission and a small number of packets each waiting for transmission, wireless resources are still allocated to the receivers with unfavorable channel quality, thereby lowering the effect of power saving.
In FIG. 21B, the threshold value is set to be high (the threshold value 2). When a high threshold value is set, wireless resources are not allocated to the receivers with unfavorable channel quality. Therefore, the effect of power saving is increased. In the case of a small number of receivers waiting for transmission and a small number of packets each waiting for transmission, a high threshold value leads to a good result. However, as shown in FIG. 21B, in the case of a large number of receivers waiting for transmission and a large number of packets each waiting for transmission, because of the increased terminals to which wireless resources are not allocated and thus transmission is not allowed, the waiting time of the transmission packets increases at a great rate, thereby giving rise to congestion.
As described hereinbefore, when the threshold value is fixed, it is difficult to sufficiently achieve the power-saving effect while restraining the occurrence of congestion.